This invention relates to composite materials including a filler and a polymer, in which the composite has low density, low microwave loss and a predetermined dielectric constant.
Microwave lenses and antennas can be made from microwave transmissive high dielectric materials, such as plastics that contain high dielectric constant fillers.
High-density inorganic compounds, such as titanium dioxide and barium titanate, can be used as fillers in the preparation of high dielectric materials for microwave applications. As the volume loading of these fillers is increased to elevate the dielectric constant of a composite material, the density of the composite material also increases. Lower density high dielectric materials have been made by coating metal onto relatively large particles of low density materials.
Microspheres are relatively small, substantially spherical particles, either solid or hollow. Hollow microspheres are also called microbubbles. Microspheres are frequently made of glass. Microspheres with a metal coating have been incorporated into polymeric articles.
Briefly, the present invention provides a composite material comprising a matrix material comprising a microwave transmissive polymer; and from about 1 to 65 volume percent of a particulate filler dispersed in the matrix. The particulate filler is characterized by an electrically conductive coating on the filler particles, particles selected from spheroids and ellipsoids having a major dimension below about 0.5 mm (preferably below about 0.2 mm) and particles having an aspect ratio greater than about 2 to 1 having a minor dimension below 100 xcexcm, and a combination of particle size and coating thickness sufficient to yield a combination of a composite material dielectric constant between about 1.2 to 100, and a composite material microwave loss tangent no greater than about 0.10 at 1 GHz.
In another aspect, the present invention provides a composite material comprising a matrix material comprising a microwave transmissive polymer; and from about 1 to 65 volume percent of a filler comprising microspheres dispersed in the matrix. The filler microspheres include an electrically conducting coating, with particularly useful electrically conducting coatings including, for example, aluminum, stainless steel, titanium, and tungsten. The filler microspheres are also characterized by: an average particle size in the range of 10 to 350 xcexcm; a density below about 3.5 g/cc; and a combination of particle size and coating thickness sufficient to yield a combination of a composite material dielectric constant between about 1.2 to 100, and a composite material microwave loss tangent no greater than about 0.10 at 1 GHz.
As used herein:
xe2x80x9cmatrixxe2x80x9d means a network of polymeric material, which may be continuous or discontinuous and may include voids and/or a gas;
xe2x80x9cfillerxe2x80x9d means coated or uncoated particles, which may be hollow or solid and which may be made from inorganic materials such as glass or ceramics or organic materials such as polymers and may be in various shapes such as spheres, fibers, and/or flakes;
xe2x80x9celectrically conductivexe2x80x9d means having a resistivity between about 10xe2x88x926 to 100 ohm-cm;
xe2x80x9cmajor dimensionxe2x80x9d means the largest of a particle""s diameter, length, width, cross-section, or thickness, and xe2x80x9cminor dimensionxe2x80x9d means the smallest of a particle""s diameter, length, width, cross-section, or thickness, where both major and minor dimensions can be directly measured or classified by standard screening techniques or particle sizing equipment;
xe2x80x9csubstantially hollowxe2x80x9d means encompassing at least some void or gas;
xe2x80x9cbubblesxe2x80x9d means substantially hollow particles, usually having a spherical shape;
xe2x80x9cmicrobubblexe2x80x9d and xe2x80x9cmicrospherexe2x80x9d mean a sphere below about 350 xcexcm particle size;
xe2x80x9cdensityxe2x80x9d means the ratio of weight to a volume of particulate material such as described in ASTM D2840;
xe2x80x9cmelt processablexe2x80x9d as applied to hollow microspheres, means capable of withstanding pressures such as typically encountered during mixing with a polymer melt or forming operations to make lenses or antennas, without crushing a majority of the microspheres;
xe2x80x9cmetal coatedxe2x80x9d means substantially encapsulated by an electrically continuous metal layer;
xe2x80x9cchemical blowing agentxe2x80x9d means a material that can be incorporated into an extrudable polymer and that decomposes at temperatures above the temperature used during extrusion, to produce a gas, such as nitrogen or carbon dioxide, so that the gas introduces voids in a polymer matrix when the material is heated above its decomposition temperature; and
xe2x80x9csignificantly rupturedxe2x80x9d means microspheres damaged to an extent that renders them ineffective as a low density filler.
It is an advantage of the present invention to provide a low density, predetermined dielectric constant, composite material with low dielectric loss in the microwave frequency region. The composite material dielectric constant can be selected within a wide range. Increasing the volume fraction of the preferred filler in the composite material increases the dielectric constant of the composite material. In some embodiments, increasing the volume fraction of filler reduces, rather than elevates, the density of the composite material. These inventive materials are useful, for example, in microwave antennae and microwave lens applications, especially where weight is an important consideration.